Those drivers are athletes.

5.5 G's around every corner. Enough to make the average person pass out. Not to mention, they are open wheel, which causes strain and stress on the body that say, NASCAR doesn't. This also goes for Indycar too, because the amount of physical exercion required to drive these cars is tremendous. Also, if you don't keep your speed up, either your tires, brakes, or both, will cool and cause diasaster.